1. Field of the Invention
The present invention relates to a technology for controlling a residual battery capacity of secondary batteries such as a nickel-metal hydride secondary battery, which are on board an electric vehicle (PEV), a hybrid vehicle (HEV) and the like, so as to perform an energy management of a system with high precision.
2. Related Background Art
Secondary batteries include a lead battery, a nickel-cadmium (Nixe2x80x94Cd) battery, a nickel-metal hydride (Ni-MH) battery, a lithium-ion battery, and the like. These batteries have a characteristic of being charged by connecting to an external power source from which a predetermined current is supplied, as electrical power is consumed. With this characteristic, these batteries have been used for various equipment.
For example, the batteries are mounted on a vehicle and serve as a battery for starting an engine, which supplies electrical power to a spark plug of the engine at the time of starting the engine. Recently, Ni-MH batteries are used also for a main power source for driving a motor of an electric vehicle and a so-called hybrid vehicle (HEV) provided with an engine and a motor.
As for HEVs, in a case where an output from an engine is larger than a power required for driving the vehicle, then surplus power is used for driving a generator so as to charge a secondary battery. Conversely, in a case where an output from the engine is smaller, then electrical power from the secondary battery is used for driving the motor so as to compensate a shortage of the power. In the latter case, the secondary battery is discharged. When mounting a secondary battery on a hybrid vehicle or the like, it is required to control such charge/discharge operations so as to maintain appropriate operating conditions.
To this end, a residual battery capacity (i.e., State of Charge (SOC)) control is conducted, where a voltage, a current, a temperature, and the like of the battery are detected, the expected residual battery capacity is estimated by an operation using these values, and a control is conducted so as to optimize a fuel consumption efficiency of the vehicle. In addition, in order to better balance a power assist operation of driving the motor during acceleration and an energy recovery operation (regenerative braking) during deceleration, generally, the SOC level is controlled so as to be in a range between 50% and 70%, for example. That is, if the SOC decreases to 50%, for example, then control for excessive charge is given. Conversely, if the SOC increases to 70%, for example, then control for excessive discharge is given, so that the SOC is brought near the middle of the control range.
Especially, in a secondary battery including as an active material a nickel oxide at a positive electrode, whose voltage variation is small with respect to a variation in the SOC, the SOC mainly is calculated by totaling discharge/charge currents in the battery, and the SOC generally is corrected using a voltage at a low SOC region and a high SOC region where a voltage variation becomes large.
However, such a correction using a voltage at the low and high SOC regions is vulnerable to a memory effect and a steep change in SOC recognition values might occur during correction. Therefore, this method is not favorable for the energy management of the system.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a method and an apparatus for controlling a residual battery capacity of a secondary battery, by which the precision in the energy management of the system can be improved substantially.
To fulfill the above-stated object, a method for controlling a residual battery capacity of a secondary battery according to the present invention includes: detecting a current flowing through the secondary battery to perform an operation on a residual battery capacity by multiplying the detected current by a predetermined charge efficiency; detecting an output voltage from the secondary battery to calculate an average voltage value of output voltages detected for a predetermined period of time; calculating an average value of residual battery capacities obtained by the operation for the predetermined period of time; referring to a reference voltage value of the secondary battery corresponding to the calculated average value of residual battery capacities; comparing the reference voltage value and the average voltage value; and setting the predetermined charge efficiency variably, based on a result of the comparison.
In this method for controlling a residual battery capacity, an average value of no-load voltages of the secondary battery is calculated for the predetermined period of time, and thus a calculated average value may be set as the average voltage value.
In the above method according to the present invention, as the result of the comparison, if the average voltage value is higher than the reference voltage value, then the predetermined charge efficiency is increased, if the average voltage value is lower than the reference voltage value, then the predetermined charge efficiency is decreased, and if the average voltage value is equal to the reference voltage value, then the predetermined charge efficiency is not changed.
In this case, it is preferable that the increase or the decrease in the predetermined charge efficiency is conducted by adding, subtracting, or multiplying a constant or a value corresponding to a difference between the average voltage value and the reference voltage value with respect to the predetermined charge efficiency.
In the above method according to the present invention, it is preferable that a temperature of the secondary battery is detected and the predetermined charge efficiency is determined based on the detected temperature and a currently calculated residual battery capacity.
In addition, it is preferable that a temperature of the secondary battery is detected and the reference voltage value is determined based on the detected temperature and the average value of residual battery capacities.
To fulfill the above-stated object, a first residual battery capacity control apparatus for a secondary battery according to the present invention includes: a current detection unit that detects a current flowing through the secondary battery; a residual battery capacity operation unit that performs an operation on a residual battery capacity by multiplying a current signal from the current detection unit by a predetermined charge efficiency; a voltage detection unit that detects an output voltage from the secondary battery; an average voltage calculation unit that calculates an average voltage value of voltage signals output from the voltage detection unit for a predetermined period of time; an average residual battery capacity calculation unit that calculates an average value of residual battery capacities obtained by the operation by the residual battery capacity operation unit for a predetermined period of time; a reference voltage storage unit that stores a reference voltage value of the secondary battery, corresponding to the average value of residual battery capacities obtained from the average residual battery capacity calculation unit; a comparison unit that compares the average voltage value obtained from the average voltage calculation unit and the reference voltage value obtained from the reference voltage storage unit; and a charge efficiency setting unit that sets the predetermined charge efficiency variably with respect to the residual battery capacity operation unit, based on a result of the comparison by the comparison unit.
To fulfill the above-stated object, a second residual battery capacity control apparatus for a secondary battery according to the present invention includes: a battery pack composed of a combination of a plurality of electric cells, each of which is a secondary battery, and used in a middle charged state, a current detection unit that detects a current flowing through the battery pack; a residual battery capacity operation unit that performs an operation on a residual battery capacity by multiplying a current signal from the current detection unit by a predetermined charge efficiency; a voltage detection unit that detects an output voltage from the battery pack; an average voltage calculation unit that calculates an average voltage value of voltage signals output from the voltage detection unit for a predetermined period of time; an average residual battery capacity calculation unit that calculates an average value of residual battery capacities obtained by the operation by the residual battery capacity operation unit for a predetermined period of time; a reference voltage storage unit that stores a reference voltage value of each secondary battery, corresponding to the average value of residual battery capacities obtained from the average residual battery capacity calculation unit; a comparison unit that compares the average voltage value obtained from the average voltage calculation unit and the reference voltage value obtained from the reference voltage storage unit; and a charge efficiency setting unit that sets the predetermined charge efficiency variably with respect to the residual battery capacity operation unit, based on a result of the comparison by the comparison unit.
In the first and the second residual battery capacity control apparatus, the average voltage calculation unit may calculate an average value of no-load voltage signals.
In addition, in the first and the second residual battery capacity control apparatus, as the result of the comparison by the comparison unit, if the average voltage value is higher than the reference voltage value, then the charge efficiency setting unit increases the predetermined charge efficiency; if the average voltage value is lower than the reference voltage value, then the charge efficiency setting unit decreases the predetermined charge efficiency; and if the average voltage value is equal to the reference voltage value, then the charge efficiency setting unit does not change the predetermined charge efficiency.
In this case, it is preferable that the charge efficiency setting unit increases or decreases the predetermined charge efficiency by adding, subtracting, or multiplying a constant or a value corresponding to a difference between the average voltage value and the reference voltage value with respect to the predetermined charge efficiency.
It is preferable that the first and the second residual battery capacity control apparatus for a secondary battery further includes a temperature detection unit that detects a temperature in the secondary battery, wherein the charge efficiency setting unit includes a charge efficiency storage unit that stores a charge efficiency corresponding to a temperature signal from the temperature detection unit and a current residual battery capacity from the residual battery capacity operation unit.
Further, it is preferable that the first and the second residual battery capacity control apparatus for a secondary battery further includes a temperature detection unit that detects a temperature in the secondary battery, wherein the reference voltage storage unit stores the reference voltage value corresponding to a temperature signal from the temperature detection unit and an average value of residual battery capacities from the average residual battery capacity calculation unit.
According to the above-stated methods and configurations, when an SOC recognized by the calculation (recognized SOC) is judged to be higher than the actual SOC, the charge efficiency is lowered. Thereby, during the subsequent integration process, the recognized SOC would be decreased from what it was, so that the recognized SOC becomes closer to the actual SOC. On the other hand, when a recognized SOC is judged to be lower than the actual SOC, the charge efficiency is increased. Thereby, during the subsequent integration process, the recognized SOC would be increased from what it was, so that the recognized SOC becomes closer to the actual SOC as well. Therefore, by continuing this control process, the recognized SOC can be controlled constantly so as to become closer to the actual SOC, so that the precision of the energy management of the system can be improved substantially.